I concur. The backup software is possibly the problem, but the solution is to issue some sort of command to abort the dump on the NTAP side, and then have the snapshots deleted (or, Heaven forbid, do it manually). I indentified this problem with backups on Netapp filers years ago. It is not just NDMP; regular dump can "hang" too. (There's also the issue about the dump having more reasonablt timeouts.)
Bruce